1. Field of the Invention
The present invention relates to a method for forming an image in an electrophotographic printer, and more particularly, to a method for forming an image in an image forming system in which a developing unit is attached and detached from a photosensitive medium and developing proceeds. The present application is based on Korean Application No. 2002-1380, filed Jan. 10, 2002, which is incorporated herein by reference.
2. Description of the Related Art
In general, an electrophotographic printer such as a color laser printer, has an image forming system in which an electrostatic latent image is formed on a photosensitive medium and then, the electrostatic latent image is developed with a toner, and a developed image passes through a predetermined transferring medium and is transferred onto a paper, and then, the developed image is heated and pressed, and the heated and pressed image is finally fixed on the paper.
FIG. 1 illustrates an example of an image forming system of an electrophotographic printer.
The image forming system includes a photosensitive drum 10 as a photosensitive medium, a charger 11 for charging the photosensitive drum 10, a laser scanning unit 12 as an exposure unit, for scanning light on the electrified photosensitive drum 10 and forming an electrostatic latent image, a developing unit 13 for developing the electrostatic latent image with a toner having four colors such as yellow (Y), magenta (M), cyan (C), and black (K), a transferring belt 14 for sequentially overlapping the four color images which are developed on the photosensitive drum 10, a first transferring roller 14a for transferring the image which is developed on the photosensitive drum 10 to the transferring belt 14, a second transferring roller 14b for transferring an image which overlaps four colors on the transferring belt 14 onto the paper, and a fixing unit 15 for heating and pressing the print and fixing the transferred image. Four-color developers 13-Y, 13-M, 13-C, and 13-K, which are included in the developing unit 13, are elastically biased by a spring (not shown) in a direction where they are separated from the photosensitive drum 10. The four color developers selectively move toward the photosensitive drum 10 through the rotation of a cam 13b, so that developing rollers 13a which are provided at the front end of the color developers contact the photosensitive drum 10. Reference numerals 16, 17, 18, and 19 denote a paper cassette, a blade for cleaning a photosensitive drum, an anti-electrostatic unit, and a transfer path through which the paper is discharged, respectively.
In the above structure, an image forming process is performed as described below. First, in a case where the photosensitive drum 10 is charged by the charger 11, the laser scanning unit (LSU) 12 scans light and thus forms the electrostatic latent image of an image which is to be developed as a first color. For example, in a case where a yellow color is developed as the first color, as shown in FIG. 2, the developer 13-Y with a yellow color is moved to the photosensitive drum 10 by the action of a corresponding cam 13b-Y and thus the electrostatic latent image formed on the photosensitive drum 10 is developed as a yellow toner.
FIG. 3 illustrates the state where the photosensitive drum 10 is in contact with a developing roller 13a-Y of the developer 13-Y. In FIG. 3, a toner T which is stained at the outside circumferential side of the developing roller 13a-Y is moved onto the portion of the electrostatic latent image of the photosensitive drum 10 contacting the toner T. In such a case, the developing roller 13a-Y rotates at a linear velocity which is faster than the linear velocity of the photosensitive drum 10 so as to increase the amount of a toner per area with which the photosensitive drum 10 is coated. The yellow image developed thereby is firstly transferred to the transferring belt 14 through a first transferring nip N1.
Next, an electrostatic latent image for a second color is formed through charging and exposure of the photosensitive drum 10. In a case where the second color is magenta, as shown in FIG. 4, the developer 13-M is moved to the photosensitive drum 10 and thus the electrostatic latent image is formed. The magenta image developed thereby is overlapped and transferred to the transferring belt 14 to which the yellow image is transferred.
A cyan image for a third color and a black image for a fourth color are developed and transferred in the same way, and thereby an image having a desired color is finally made on the transferring belt 14.
After that, the color image made thereby is transferred onto the paper which is supplied to a second transferring nip N2 between the transferring belt 14 and the second transferring roller 14b. The transferred image passes through the fixing unit 15, where it is heated and pressed and thus is completely fixed on the paper.
However, in the above configuration, since the four-color developers 13-Y, 13-M, 13-C, and 13-K of the developing unit 13 forms images while being alternately attached and detached from the photosensitive drum 10, the photosensitive drum 10 may be shocked whenever each of the developing rollers 13a of each of the developers 13-Y, 13-M, 13-C, and 13-K collides with the photosensitive drum. In such a case, jitter errors caused by image tremble during collision may occur. However, a problem arises in that each of the developing rollers 13a of each of the developers 13-Y, 13-M, 13-C, and 13-K rotates at a linear velocity which is faster than that of the photosensitive drum 10 as mentioned above, and thus shock caused by a difference in the linear velocity occurs during contact, and thereby the shock applied to the photosensitive drum 10 is further increased.
That is, a photosensitive layer is usually coated on an aluminum material to form the photosensitive drum 10, and the developing rollers 13a have a rubber material of predetermined hardness. Thus, in a case where the hardness of the rubber material is lowered, shock during contact may be slightly alleviated. However, the difference in the linear velocity described above is set so that the toner is sufficiently supplied to the photosensitive drum 10. In a case where the developing roller 13a and the photosensitive drum 10, which have different linear velocities, contact each other, the developers 13-Y, 13-M, 13-C, and 13-K are instantaneously pushed during contact, and thus registration defects in which the overlapping position between colors crosses each other, as well as the jitter errors occur, and thereby an image is not clear and abnormally developed.
In addition, due to shock during contact, the toner scatters and thus contaminates peripheral devices. The problem caused by the difference in linear velocities may occur during break off as well as during contact. In a case where the velocity of the developing roller is reduced continuously, the supply of the toner is insufficient during development, and thus image quality may be lowered.
Thus, for developing of a clear image, a method is needed in which the amount of the toner supplied to the photosensitive drum 10 during development is not reduced, and the above causes for the occurrence of defective images are eliminated.
To solve the above problems, it is an object of the present invention to provide a method for forming an image in an electrophotographic printer which is improved so that shock is alleviated when a photosensitive medium and a developer contact each other.
Accordingly, to achieve the above object, there is provided a method for forming an image in an electrophotographic printer, which is capable of removing a developing roller of a developer from a photosensitive medium on which an electrostatic latent image is formed, and of developing the electrostatic latent image with a predetermined color. The method includes the steps of rotating a developing roller at the same linear velocity as that of a photosensitive medium in a state where the developing roller is separated from the photosensitive medium, moving a developer so that the developing roller is in contact with the photosensitive medium, and changing the linear velocity of the developing roller to be faster than that of the photosensitive medium after contact and then proceeding with the developing process.
Here, in order to alleviate shock caused when the developing roller is detached, it is preferable that the method further includes the steps of rotating the developing roller at the same linear velocity as that of the photosensitive medium after completing developing, and moving the developer so that the developing roller is separated from the photosensitive medium.